1. Field of the Invention
This invention relates to large, or wide, aperture photographic objectives, particularly in the 50 mm focal length, F/1.2 class, and more particularly, to objectives using an aspherical surface for maintaining a long back focus and providing extremely good correction for flare at the fully open aperture, and also providing good correction for spherical aberration, field of curvature, and coma.
2. Description of the Prior Art
A Gauss type lens produces flare (sagittal flare) mainly because the forward and rearward lens surfaces adjacent the diaphragm are strong in negative curvature. However, limiting the curvature of these surfaces causes rapid deterioration in quality due to spherical aberrations and field curvature.
The index of refraction of the positive lens may be increased to facilitate aberrational corrections. However, optical glasses having the high refractive indices required for this purpose are not available at the present time. Therefore, in F/1.2 class lenses, effective removal of flare at fully open aperture, spherical aberration, and curvature of field becomes difficult with a spherical system.
An objective which corrects for flare and various aberrations using an aspherical surface is disclosed in Japanese Patent Publication No. Sho 47-19386, assigned to the assignee of the present invention.